Existing heat exchangers generally comprise a shell (a large vessel) having a bundle of tubes (commonly referred to as a tube bundle) positioned within a cavity of the shell. Two fluids of different starting temperatures flow through the heat exchanger. One fluid, known as the tube-side fluid, flows inside of the tubes of the tube bundle. A second fluid, known as the shell-side fluid, flows through the cavity of the shell on the outside of the tubes. The fluids may both be liquids or they may both be gases. Alternatively, one of the fluids may be a gas while the other fluid is a liquid. During operation of a typical heat exchanger, heat is transferred between the two fluids without direct contact between the two fluids. Specifically, heat is transferred from the hotter fluid, through the walls of the tubes, and into the cooler fluid. The transfer of heat without contact between the shell-side fluid and the tube-side fluid is particularly desirable in the nuclear power plant industry because the primary or secondary fluids may become radioactive. Depending upon the fluids used and the desired results, heat is transferred either from the tube-side fluid to the shell-side fluid, or vice versa.
A typical solar power plant uses a preheater, a steam generator and a superheater to produce steam for introduction into a turbine where that steam is converted into useful work. In such a system, hot oil is typically used as the tube-side fluid and water (in either liquid or vapor form) is typically used as the shell-side fluid. A steam generator is a heat exchanger that serves to transfer the thermal energy of hot oil to liquid water to convert the liquid water to steam. The tube bundle is submerged in the liquid water (or other shell-side fluid) during the transfer of heat into the shell-side liquid. As the water (or other shell-side fluid) is converted into steam, it is replenished by introducing additional pre-heated feedwater into the shell-side chamber.
When the cavity of the shell is full of the liquid water and the tubes are filled with the hot oil, the liquid water surrounding the tubes of the tube bundle reaches its boiling point, thereby creating steam bundles. The process of the liquid water being converted into steam bundles is known as nucleate boiling. In existing steam generators, a deficiency known as vapor blanketing is prevalent. Vapor blanketing occurs due to the high surface tension of steam, which causes the liquid water to be unable to make surface contact with the outer surface of the tubes of the tube bundle. In other words, due to its high surface tension, the steam hugs the outside surfaces of the tubes and forms a vapor blanket, or a barrier of air, around the outer surface of the tubes that is impenetrable by the shell-side liquid water. Since the shell-side liquid water is unable to make contact with the tubes at areas having vapor blanketing, the temperature of the tubes exceeds the thermal capacity of the shell-side liquid water, thereby creating a hot spot.
Vapor blanketing typically occurs deep inside the tube bundle because the shell-side liquid water (or other shell-side fluid) is unable to reach the outer surfaces of the tubes that are located deep within the tube bundle fast enough to keep the outer surfaces of those tubes wet. Vapor blanketing inhibits heat transfer and results in a reduced heat exchanger performance.
Previous attempts to address the vapor blanketing problem have been ineffective and inefficient. For example, a common remedy to vapor blanketing is to use a more open tube layout with a larger pitch-to-diameter ratio of the tube bundle. This remedy requires the use of a much larger shell and therefore results in a much higher equipment cost. Thus, a need exists for an apparatus, method and/or system that eliminates the potential for vapor blanketing while not increasing the costs of manufacturing.